A "twisting ball" sheet medium for displaying an image wherein light falling on the medium, under an applied voltage, forces internal bichromal balls to rotate to show either the white or black side.
U.S. Pat. Nos. 4,126,854 and 4,143,103 describe a twisting ball display and are incorporated by reference herein. These patents describe a display system in which the display panel is comprised of spherical particles, balls, which have an optical and electric anisotropy due to each hemisphere surface having a different color and electrical charge. These particles are imbedded in a solid substrate and a slight space between each ball and the substrate is filled with a fluid so that the balls are free to rotate, in a changing electrical field, but can not migrate from one location to another. If one hemisphere is black and the other white, each pixel can be turned on or off by the electrical field applied to that location. Each pixel can be individually addressed by applying an electrical field that is greater than the threshold field required for ball rotation, and a full page image can thus be generated.
This medium is fabricated by first mixing a large number of balls into a solid substrate, curing the substrate, and then soaking the resultant substrate in a plasticizer. The substrate will swell, creating a space around each ball, and the space will fill with the liquid plasticizer, thus allowing each ball to rotate under the effect of an applied electrical field. The result is reusable "electric paper" which can be used, for example, in a raster output scanner, to produce an image from a bit map of electrical fields, or can be used as the flat panel display of a terminal or notebook computer.
It would be useful if this substrate could be modified to produce images from exposure to electromagnetic radiation images, either visible light or x-rays. One way of accomplishing this would be to create a sandwich structure in which this substrate is placed adjacent to a photoconductor layer, with radiation transparent electrodes at the distal surfaces. Upon complete exposure of an area of the photoconductor layer the fraction of the voltage that was across that layer would be collapsed onto the adjacent area of the twisting ball substrate, so that the entire voltage across the sandwich structure would now be across this area of substrate. If this voltage now exceeds the threshold voltage the bichromal balls in that area will rotate.
The problem is that typical threshold switching behavior requires that the photoconductor layer and the substrate have about the same capacitance, which is proportional to the dielectric constant of the layer divided by the actual thickness of the layer. The substrate material has a dielectric constant of about 2.75 and a photoconductor like selenium has a dielectric constant of about 6. This means that if we use a selenium photoconductor to address our twisting ball substrate it must be about twice as thick as the twisting ball substrate. For a twisting ball substrate using 100 micron diameter balls we will have a minimum thickness of about 150 microns. A 300 micron thick photoconductor suitable for addressing the twisting ball display is very thick and expensive and generally not desireable.
It would be useful, then, to photosensitive the twisting ball display in a more desireable manner. In doing this in the manner of this invention we will find other desireable properties that have further value.